The deep divisions within the US Democratic party were on clear display on the first day of the party convention in Philadelphia on Monday.

Supporters of Bernie Sanders chanted his name and booed mentions of Hillary Clinton. That was despite Sanders calling for unity and saying that Clinton will make an “outstanding” president.

“Any objective observer will conclude that, based on her ideas and her leadership, Hillary Clinton must become the next president of the United States,” Sanders told the convention.

Sanders finally endorsed Clinton on 12 July after months of bitter battling against her for the Democratic nomination for president.

In that time, he galvanised a movement of anti-establishment voters and attacked his Clinton’s links to big business.

“I don’t think I have to explain to the American people what Wall Street did to this economy, and yet in her last filing report for her major super PAC, one of several, she raised $15 million from Wall Street,” Sanders said in April.

In a February speech, he said of Clinton: “Now, she’s a very good speaker, I admit that. But to to get 225,000 dollars for a speech to Goldman Sachs, you’ve got to be really good. I don’t know that she’s that good.”

Even though Sanders has endorsed Clinton, some of his supporters are still not prepared to stop backing their man, arguing that the primaries were skewed in Clinton’s favour.

“The super-delegates are being bought and the Democratic Party is being hijacked by the oligarchy that is corrupt and it stretches form the Clintons on down. The Democratic Party stands for the right things it is being hijacked by the wrong people,” said Sanders delegate Alexis Adelstein.

Pro-Sanders protester said: “We don’t have to say – ‘Oh well, we will vote for Hillary against Trump.’ Nominate the person who can beat him. Bernie beats him in poll after poll.”

The Trump danger for the Democratic party is real – and could become yet more pronounced if the party cannot heal its divisions.

Trump was formally named the Republican presidential nominee last week.

A new opinion poll, from CNN-ORC, has given him a 3 percent lead over Clinton.